1) Field of the Invention
The present invention relates to an image display apparatus including a current-controlled light emitting diode that emits light based on electric current supplied by a current source, and more specifically, relates to an image display apparatus having a configuration such that the potential of a wiring structure connected to the current source is changed.
2) Description of the Related Art
An organic light emitting diode (hereinafter, “organic LED”) display apparatus using an organic electroluminescent (EL) device that emits light itself, is most suitable for making the apparatus thin, since it does not require a backlight, which is required in a liquid crystal display apparatus, and does not have any limitation in the angle of visibility. Therefore, practical use thereof is expected as a next-generation display apparatus.
As the image display apparatus using the organic LEDs, a simple (passive) matrix type and an active matrix type are known as the drive system. The former has a simple configuration, but has a problem in that realization of a large-scale and highly delicate display is difficult. Therefore, development of the active matrix type display apparatus has been recently performed, which controls-the current flowing through light emitting diodes in pixels, by an active element provided in the pixel, for example, a thin film transistor.
FIG. 5 depicts a pixel circuit in the conventional active matrix type organic LED display apparatus. The pixel circuit according to the conventional apparatus includes an organic LED 105, being a current-controlled light emitting diode, a thin film transistor 104 whose drain electrode is connected to the negative electrode of the organic LED 105 and whose source electrode is connected to a wiring structure 108, and serving as a driver element, a capacitor 103 connected between a gate electrode of the thin film transistor 104 and the wiring structure 108, and a thin film transistor 102 whose drain electrode is connected to the gate electrode of the thin film transistor 104, source electrode to a signal line 101, and the gate electrode to a scan line 106, respectively, and serving as a switching element. The organic LED display apparatus has a current source 107 for supplying electric current flowing through the organic LED 105, and the current source 107 has such a structure that it is electrically connected to the thin film transistor 104 via the wiring structure 108.
In the pixel circuit shown in FIG. 5, a voltage corresponding to the display brightness is supplied from the data line 101 to the capacitor 103 via the thin film transistor 102. Since the capacitor 103 is arranged between the gate and the source of the thin film transistor 104, the gate to source voltage of the thin film transistor 104 becomes equal to the voltage stored in the capacitor 103, and a predetermined channel is formed between the source and the drain based on the gate to source voltage. The current source 107 supplies the electric current corresponding to the mobility realized by the channel of the thin film transistor. 104, so that the current flows to between the source and the drain of the thin film transistor 104 and the organic LED 105 serially connected to the thin film transistor 104, and the organic LED 105 emits light with desired brightness (See Japanese Patent Application Laid-Open No. H8-234683 for example).
An image display apparatus, in which a compensation circuit that compensates threshold voltage fluctuations in the thin film transistor 104 is incorporated, is also known. It is preferable to use amorphous silicon for the channel forming area of the thin film transistor 104, in order to suppress fluctuations in the IV characteristics of the driver element for each display pixel. When the amorphous silicon is used, however, it is known that the threshold voltage fluctuates due to longtime use, and it is desired to compensate the threshold voltage fluctuation from a viewpoint of enabling high quality image display.
There are various configurations of the compensation circuit, and as one example, a configuration in which a thin film transistor for voltage compensation is arranged, and voltage compensation is performed by combining the operation of such a thin film transistor and potential changes of the wiring structure 108 is known. When such a compensation circuit is arranged, the current source 107 not only performs a function of supplying electric current to the organic LED 105, but also operates for changing the potential of the wiring structure 108 by supplying an electric charge to the wiring structure 108.
However, the image display apparatus using the organic LEDs has various problems due to the structure in which the current is supplied to the organic LEDs at the time of image display. In the actual image display apparatus, it is necessary to increase the physical length of the wiring structure 108 with respect to the display pixel arranged away from. the current source 107, and it is necessary to increase the sectional area of the wiring structure 108, in order to suppress an increase in the electrical resistivity with an increase in the physical length.
On the other hand, due to the increase in the sectional area of the wiring structure 108, the area in which the wiring structure 108 overlaps on another wiring structure, for example, the scan line 106 increases, thereby increasing the parasitic capacitance of the wiring structure 108. The problem due to the parasitic capacitance is elicited in the configuration in which the potential of the wiring structure 108 is changed, for example, when the compensation circuit is incorporated in the image display apparatus.
For example, when the threshold voltage fluctuation of the thin film transistor 104 is compensated by incorporating the compensation circuit, it is necessary to change the potential of the wiring structure 108 at the time of operation. In order to change the potential, it is necessary to supply electric charges with respect to the parasitic capacitance. Therefore, when the parasitic capacitance of the wiring structure 108 increases, the time required for changing the potential increases corresponding to the increased amount of the parasitic capacitance.
An increase in the time required for the potential change means that the time required for voltage compensation also increases, leading to restrictions on achieving high definition or a large screen of the image display apparatus. That is, while compensation for the threshold voltage fluctuation is required for all driver elements provided for the respective pixels, the time allowed for performing the voltage compensation with respect to all driver elements is limited to a certain value. Therefore, in order to increase the number of pixels in view of realizing high definition or a large screen of the image display apparatus, it is essential to reduce the time required for voltage compensation with respect to the individual driver element.
Power consumption of the current source 107 required at the time of changing the potential of the wiring structure 108 is another problem. Since it is normal that the compensation circuit operates for each frame, the current source 107 needs to supply the current with respect to the wiring structure 108 for each frame separately from the light emitting phase, in order to change the potential of the wiring structure 108. Since certain electrical resistivity and parasitic capacitance exist in the wiring structure 108, it cannot be avoided that a certain amount of power consumption occurs in the current source 107, with a potential change of the wiring structure 108. When such power consumption is small, there is no problem, but actually, unignorable amount of power consumption is required, and it is concerned that the heat generated from the current source 107 may adversely affect the image display apparatus and the current source 107 itself.